Water deep below ground has safely trapped carbon dioxide for millions of years and may one day help absorb emissions of the greenhouse gas to help slow climate change, researchers said.

The finding shows that such carbon capture and storage is possible provided scientists find an area where the geology is suitable, said Chris Ballentine, a researcher at the University of Manchester, who worked on the study.

This means locating ancient deep water systems thousands of metres below the surface to ensure gas doesn't escape back to the surface and into the atmosphere, he said.

'Clearly we want to bury carbon dioxide in the ground, that is a no-brainer,' Ballentine said. 'The big question is when we put carbon dioxide into the ground, how safe is it?'

The world is looking to limit emissions of greenhouse gases like CO2 as climate scientists warn that their elevated global levels will lead to higher temperatures, rising seas, drought, and cause floods, heat waves and stronger storms.

Many governments see carbon capture and storage as a key weapon in the fight against global warming because it captures the emissions from fossil fuel burning power stations and buries them underground, in a process which could keep up to a third of all carbon emissions out of the atmosphere.

However, the technology is untried at a commercial scale and will initially be very expensive, at around 1 billion euros per power plant, making it unattractive for individual companies to undertake without support.

But the prize for the winner is potentially vast, with China on its own opening one coal-fired power plant a week and global reserves of coal which could last hundreds of years.

Ballentine and colleagues analysed how carbon dioxide dissolved into water and another technique to see if it reacted with the rocks at nine natural gas fields in North America, China and Europe filled with the greenhouse gas thousands or millions of years ago following volcanic eruptions.

They found that underground water was the major carbon sink in these gas fields and had been for millions of years, potentially offering vast areas to store the greenhouse gas one day, the researchers said.

While other studies have shown that certain rocks below the surface soak up carbon, the findings published in the journal Nature found that most rocks do not store the greenhouse gas and the water instead keeps it safe.

'By combining two techniques, we've been able to identify exactly where the carbon dioxide is being stored for the first time,' Stuart Gilfillan, a researcher at the University of Edinburgh, who worked on the study, said in a statement.

'Our study clearly shows that the carbon dioxide has been stored naturally and safely in underground water in these fields.'-Reuters